The goal of this grant proposal is to elucidate the molecular mechanisms underlying the severe congenital muscular dystrophy type Ullrich (UCMD) and its milder counterpart Bethlem myopathy (BM). UCMD and BM combine features of muscular dystrophies and disorders of connective tissue resulting in significant weakness, progressive contractures, and joint laxity. Collagen VI is a microfibrillar component found in the extracellular matrix of most tissues, often in a pericellula r distribution . Its basic heterotrimeric unit is composed of three alpha chains. Mutations in the collagen VI genes (COL6A1, COL6A2, COL6A3) have been identified in BM and more recently also in UCMD. Althoug h BM usuall y is caused by dominan t mutations and UCMD by recessive mutations, we recently demonstrated dominan t negative mechanisms in UCMD also. The relevant functions of collagen VI are unclear, in various culture systems it has been shown to promote cell adhesion, differentiation , proliferation , and survival , whereas in a mouse model of Col6al inactivatio n there was evidence for increased apoptosis in the muscle. We hypothesize that fibroblasts are the primary cell type involved in UCMD and BM causing disease both in muscle (weakness) and in tendon (laxity and contractures) by producing an abnormal extracellular matrix (ECM). We further hypothesize that a specific disturbance of the interface between muscle and its extracellular matrix (ECM) leads to muscle apoptosis and weakness. In this project we will comprehensively characterize patients with UCMD and BM to define their clinical, morphological, and biochemical phenotypes and correlate these with the collagen VI genotypes. This will enable us to delineate the entire phenotypical, biochemical, and genetic spectrum of UCMD/BM. We wil l next determine changes in ECM gene expression as well as apoptosis in dermal fibroblasts cultures from the characterized patients with mutations in the collagen VI genes. Defining changes in ECM composition and fibroblast viability will allow us to understand the basis of the congenital and progressive contractures, the joint laxity, and to predict the nature of the abnormal ECM deposited in muscle. To study the molecular basis of the muscle weakness we are developing a novel co-culture system in which various patient derived fibroblast cultures supply an ECM for normal muscle cells. This system will be used to examine the effects of normal and mutant collagen VI matrices on the myotubes in culture. It will allow us to define the pathways that lead to muscle dysfunction and muscle cell death. Ultimately, we hope to understand the influence of ECM on the development of muscular dystrophy. Knowledge about these pathways is a prerequisite for developing effective treatment strategies in this new and important group of muscle disorders.